1. Field of the Invention
The present invention relates to a method and apparatus for image formation and more particularly to a method and apparatus for forming a multicolor image by electrophotography.
2. Description of the Prior Art
While there have so far been proposed many systems and apparatus for the purpose of providing multicolor images by electrophotography, these are classified broadly into the following groups. One of them is such a system that the operation of forming a latent image and developing the same with color toners on a photosensitive member is repeated for the number of times corresponding to the number of separated colors, and the colors are superimposed on the photosensitive member or transferred to a transfer material each time the development is made and superimposed on the transfer material. The other is such a system that an apparatus having a plurality of photosensitive members corresponding in number to the number of separated colors are used and light images of all the colors are simultaneously exposed on the respective photosensitive members, the latent images formed on the respective photosensitive members are developed by color toners, and these are successively transferred to a transfer material, and thus, the colors are superimposed thereon and a multicolor image is provided.
In the first system, the process of formation of the latent image and development must be repeated for a plurality number of times, and therefore, it is a great disadvantage of this system that it takes a long time for image recording and its speed-up is very difficult. And there is another disadvantage in the case of the system in which the toner images are superimposed on the photosensitive member, because the potential at the portion previously developed with one toner attached thereto is not sufficiently lowered, another toner for the later developed portion is attached to that portion already developed with that toner--where this toner should not be attached to--and therefore, color turbidity is liable to be produced.
In the case of the second system, there is an advantage that high-speed processing is made possible by the parallel use of a plurality of photosensitive members, but since a plurality sets of photosensitive members, optical systems, developing means, and so on are required, this system has a disadvantage that the apparatus becomes complex, larger, and expensive, and so, it is less practical.
And in either of these systems, there is a great disadvantage that it is difficult to register the images at the times of the repeated image formation and transfer, and it is therefore impossible to completely remove the shear in the superimposition of colors.
To thoroughly solve these problems, it is considered to provide a system that will make a multicolor image recorded on a single photosensitive member by one time of image exposure, but a method to effectively achieve such a system is not yet developed at the present time. Specifically, the developing conditions in making development by various colors of toners are not yet investigated, and it is the present situation that disturbance of toner images, insufficiency of image density, and so on cannot be avoided.
To fundamentally solve these problems, the present inventor earlier invented an apparatus capable of forming a multicolor image by one time of image exposure on a photosensitive member. The apparatus, using an electric conductive member, a photoconductive layer, a photosensitive member having a layer including a plurality of different kinds of filters, forms a multicolor image as described below. That is, by applying the surface of the above mentioned photosensitive member with electric charging and an image exposure, an image is formed by charge density on the boundary surface between an insulating layer and the photoconductive layer, then by applying the surface having the image thereon with a uniform exposure of the light of a specific color, a potential pattern is formed on the photosensitive member at the portion of the relative filter, and by developing the potential pattern with a developing device containing a toner of a specific color, a single color toner image is formed. After smoothing the potential, by applying a uniform exposure with the light that is transmitted through the filter portion different from that previously used and by development with another developing device containing a toner of a different color from the previous color, a toner image of the second color is formed on the photosensitive member. Thereafter, the potential smoothing, uniform exposure, and development are repeated for a required number of times. As a result, various colors of toners are attached to various filter portions on the photosensitive member, and thereby, a multicolor image is formed (refer to Japanese Patent Application No. 59-83096). According to this type of multicolor image forming apparatus, the image exposure is made only once, and therefore, there is really no possibility of occurrence of the shear in the different color images.
The present inventor, however, has found after investigations that, although the above mentioned multicolor image forming apparatus solved the problems that the prior art apparatus had, there still remain the following problems.
That is, in the above described apparatus, the image exposure is made from behind the charging device while it is discharging, and therefore there is produced restriction as to the designing of the apparatus. And, since the charging and image exposure are performed simultaneously, electrons or holes must be moved in the surface layer of the photosensitive member in a short time, and so, such a material that will provide a high transfer speed must be used for the photoconductive layer. The photoconductive layers of such inorganic substance as CdS and Se-Te in general provide high transfer speed for the electrons or holes, while speeds provided by the photoconductive layers of organic substance are slower. Thus, selection of materials of the photoconductive layer receives restriction.
On one hand, when an image exposure is applied through a specific filter portion and then a uniform exposure by the specific light is made, the potential produced at the specific filter portion becomes substantially equal to the background potential caused by such as the recharging.
On the other hand, there is a potential rise to be produced at other filter portions due to dark decay of the photosensitive member. Therefore, there occurs such a problem at the time of development being performed under such a condition that the specific filter portion at low potential is developed that the toner is attached also to other filter portions thereby producing color mixing. And, if the developing bias is set to a condition that will not cause the color mixing, only such a copy is obtained that is poor in gradation and full of highlights.
The present inventor has made this invention after strenuous studies to solve the problems still remained unsolved with the multicolor image forming apparatus of the above mentioned Japanese Patent Application No. 59-83096.
As a method to form an image by electrophotography, there is known a method, called the NP method (Japanese Patent Publication No. 42-23910) such that a photosensitive member formed of a photoconductive layer and a transparent insulating layer piled on a conductive substrate is applied with primary charging and the same is then subjected to a charge elimination (a secondary charge) while being applied with an image exposure, whereby a primary latent image is formed by charge distribution while the surface potential on the photosensitive member is made even, and then the same is subjected to a whole surface exposure, and thereby, a potential pattern as a secondary latent image is formed on the surface of the photosensitive member, and this secondary latent image is developed by a toner.
According to the above described method, the image exposure is made from behind the charging device at the time of the secondary charging, and so, the apparatus receives restriction as to its designing. And, since the image exposure and the secondary charging are performed simultaneously and a potential pattern is thereby formed on the surface of the insulating layer, it becomes a requisite for high sensitivity that the electrons or holes produced in the surface layer of the photosensitive member are moved to the substrate in a short time, and therefore such photosensitive materials that will provide high transfer speeds of the utilized electrons or holes must be used. Generally, photoconductive layers of inorganic substance such as CdS and Se-Te provide higher transfer speeds of the electrons or holes, whereas photoconductive layers of organic substance in general provide slower speeds. Thus, selection of the materials for the photoconductive layers receives restriction.
According to the method in which secondary charging is performed subsequent to primary charging and then an image exposure is made (Japanese Patent Laid-open No. 53-76035), the above mentioned problem is solved because the charging and the image exposure is made separately. But, the charges injected to the boundary layer on the insulating layer at the primary and the secondary charging and trapped therein are subject to the electric field opposite to that applied thereto at the time of the injection of the charges. Therefore, when the injected amount of the charges or the trapped amount of the charges suffers a change (for example, the change in the injecting or trapping performance due to a temperature change or aging of the photoconductive layer), the surface potential at the portion subjected to irradiation of the light of the image exposure (the portion will hereinafter be called the white ground portion) becomes unstable. On the other hand, at the portion not subjected to the light of the image exposure (the portion will hereinafter be called the black ground portion), the potential is stabler than the irradiated portion because the portion is controlled to be at a constant surface potential by the secondary charging. Under such conditions, if developing is made to attach a toner to the black ground portion, the toner tends to attach also to the white ground portion where the potential is unstable and thus a fog is produced. Therefore, the combination of the present image forming method and the normal development is liable to be affected by the change in the characteristics of the photosensitive member and is not considered preferable.